The overall objective of this research is to contribute to understanding of the structure and dynamics of chromatin--in particular, how that structure can change in response to the stresses involved in processes like transcription and replication. A body of knowledge fundamental to such understanding is data on the thermodynamic stability of the nucleosome, the unitary element of chromatin structure. Such information will be sought through physical studies of reconstituted systems, using cloned DNA molecules carrying "positioning" sequences and purified histone oligomers. The association-dissociation equilibria of DNA with the (H3-H4)2 tetramer will be first studied, followed by analysis of the reaction of the complex so formed with (H2A-H2B) dimers. Comparative stability studies will examine the effects of different regions in the DNA sequence on stability. Finally, a series of experiments utilizing different topological forms of a very small "minichromosome" (containing three nucleosome sites) will be directed toward an analysis of the quantitative effects of superhelical tension on nucleosome stability.